Page 117 - Understanding Automotive Electronics

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4 MICROCOMPUTER INSTRUMENTATION AND CONTROL

Information is sent to or Data is sent to the CPU over a data bus (Figure 4.2). The data bus is
received from memory slightly different from the address bus in that the CPU uses it to read
locations and input/out- information from memory or peripherals, and to write information to memory
put devices via the bidi- or peripherals. Signals on the address bus originate only at the CPU and are sent
rectional data bus. to devices attached to the bus. Signals on the data bus can either be inputs to or
outputs from the CPU that are sent or received at the CPU by the data register.
In other words, the data bus is a two-way street while the address bus is a one-
way street. Another difference is that the data bus in many microcomputers is
only 16 bits wide whereas the address bus is typically 32 bits wide.

Memory Read/Write
The CPU always controls the direction of data flow on the data bus
because, although it is bidirectional, data can move in only one direction at a
time. The CPU provides a special read/write control signal (Figure 4.2) that
tells the memory in which direction the data should flow. For example, when
the read/write (R/W) line is high, the CPU reads information into a memory
location.
During a memory read The timing diagram for a memory read operation is shown in Figure 4.3.
operation, the CPU Suppose the computer has been given the instruction to read data from memory
changes the state of the location number 10. To perform the read operation, the CPU raises the R/W line
read/write line and puts to the high level to tell the memory to prepare for a read operation. Almost
the appropriate address simultaneously, the address for location 10 is placed on the address bus (“address
on the address bus. This valid” in Figure 4.3). The number 10 in binary (0000 0000 0000 1010) is sent to
causes the contents of the memory in the address bus. The binary electrical signals corresponding to 10
the addressed memory operate the specific circuits in the memory to cause the binary data at that
location to be placed on location to be placed on the data bus. The CPU has an internal register that is
the data bus, where it activated during this read operation to receive and store the data. The data is then
can be read by the CPU. processed by the CPU during the next cycle of operation according to the
relevant instruction.
Timing
Microcomputers use a A certain amount of time is required for the memory’s address decoder to
timing signal, called a decode which memory location is called for by the address, and also for the
clock, to determine when selected memory location to transfer its information to the data bus. To allow
data should be written to time for this decoding, the processor waits a while before receiving the
or read from memory. information requested from the data bus. Then, at the proper time, the CPU
opens the logic gating circuitry between the data bus and the CPU data
register so that the information on the bus from memory location 10 is latched
into the CPU. During the memory read operation, the memory has temporary
control of the data bus. Control must be returned to the CPU, but not before
the processor has read in the data. The CPU provides a timing control signal,
called the clock, that tells the memory when it can take and release control of
the data bus.

104 UNDERSTANDING AUTOMOTIVE ELECTRONICS